WASHINGTON, Jan 29 : Many factors influence how long you live, such as diet, exercise, smoking, drinking, environment and other variables. It also helps not to get hit by a dump truck. But what about your genes? That has been a contentious question for decades. A new study points to a larger role for genetics than previous research had indicated, estimating the contribution of genes to determining human lifespan at about 50 per cent. That is roughly double what prior research concluded, and it